Pretty much the only time you’ll hear someone mention the canon in the year of our Lord 2019 is to explain why it’s bullshit: the canon is a bunch of stuff made by old or dead white dudes that a bunch of other old or dead white dudes decided was important, and everything outside of the canon is deemed, by implication, not important or worthwhile or particularly good. The canon is the epitome of cultural elitism; any English undergrad can tell you all about it.
The idea of a canon comes from the Bible, with the books deemed good, important and true being preserved and assembled as part of the Biblical canon, and other writings – like the gospel where the cross is a character that talks, or ones about Jesus as a kid – getting left on the cutting room floor. The idea of a literary canon is a kind of outgrowth from this: collecting the good and important works of literature – Homer, Dante, Chaucer, Shakespeare – as the ones worthy of study, the ones any educated person should be expected to have read. The literary canon is the stuff you’re supposed to read in school or college, but probably didn’t. There are tons of very legitimate criticisms of what makes up the literary canon: it tends to be disproportionately male – Jane Austen, the Bronte sisters, and Virginia Woolf would be the big exceptions when it comes to novelists – and almost exclusively white, and the people who decide what gets deemed canonical (academics and critics) have similar demographic problems. But the big difference between the Biblical canon and the literary canon is that there is no official list of classic books, with everything else likely to be lost or destroyed. The literary canon is necessarily in flux. When Herman Melville died, he was an obscure writer living in poverty, but a few decades later some hip literary types in New York realised no, wait, Moby-Dick is really good, actually, and now here we are.